Heretofore, lead steel has been used as a steel material having free-cutting properties. However, since lead free-cutting steel contains Pb, the problem of environmental contamination has arisen, and recently, the use of lead free-cutting steel has been restricted. For this reason, the development of new iron and steel materials posing no environmental problems which replace lead free-cutting steel is desirable.
Under such a situation, although grarphite-dispersed material is known as an iron and steel material having free-cutting properties, it has not been considered as a material having sufficient properties as free-cutting steel, because conventionally considered graphite steel has poor mechanical properties, such as, strength and processability. Therefore, the development of graphite steel which has fine graphite particles evenly dispersed therein and which has free-cutting properties and good mechanical properties is desirable.
In general, it is known that graphitization proceeds readily in high-carbon steel having a hyper-eutectoid composition. Specifically, it has been disclosed that the addition of 1% or more silicon significantly accelerates graphitization. For example, see: Tomo-o Sato et al., "Study on Graphitic Steel (1.sup.st Report) Effect of Silicon on Graphitic Steel", J. Jpn. Inst. Met., 1956, Vol. 20, pp. 5-9.
Similarly, it has been disclosed that the graphitization of high-carbon steel is accelerated by quenching, cold working, or by the addition of Al, Si, Ni, Ti, Zr, or B. For example, see: Naomichi Yamanaka et al., "On the Mechanism of Graphitization of High-Carbon Steel", Iron & Steel, 1962, Vol. 8, pp. 946-953.
The Yamanaka reference discloses that the addition of Ti causes acceleration of graphitization since the distribution of Ti in cementite is quite low, and special carbide, TiC, is easily formed. Thus, graphitization is accelerated by the effect of denitrification by Ti rather than the effect of stabilization of cementite.
However, all of these examples are high-carbon steels. For higher strength and better processability, a medium-carbon steel having a C concentration of 1.0% or below is required. Therefore, the above examples are not satisfactory or suitable as a free-cutting steel.
On the contrary, it is the present state that the graphitization of medium-carbon steels, especially hypo-eutectoid steel is difficult, and that the behaviors of graphitization by heat treatment or the addition of alloys have not been well known.
As the previously mentioned Sato reference discloses, although Si is added as an element for accelerating graphitization, it is considered that the quantity of Si is preferably 1.5% or less, since a large quantity of added Si significantly lowers ductility by solid solution hardening.
Under such a situation, there is a report describing the effects on a graphitization phenomenon of addition of Si and Ni, or Si and Co, together to hypo-eutectoid steel. For instance, see: Hidekazu Sueyoshi et al., "Effect of Alloying Elements on the Graphitization of Hypo-Eutectoid Steel", J. Jpn. Inst. Met., 1979, Vol. 43, pp. 333-339.
However, considering that the materials containing Ni or Co are difficult to recycle and that such added elements are relatively expensive, the addition of these materials is not effective, and it cannot be considered that the process for manufacturing hypo-eutectoid steel in which fine graphite particles are dispersed in a stable manner and with high reproducibility has been established.
It has been known that the graphitization of high-carbon steel is accelerated by cold working before the graphitization treatment. As one of such methods there has been disclosed a method for improving softness and ductility as low-carbon steels to hypo-eutectoid steels. For instance, see: Atsuki Okamoto, "Graphite Formation in High-Purity Cold-Rolled Carbon Steels", Metallurgical Transactions A, October 1989, Vol. 20A, pp. 1917-1925.
By this disclosed method, it is considered that cold rolling is conducted until cementite is divided, and the gaps of dividing (eg. voids) become sites of graphitization.
However, this example has problems in that stringent cold working of 20% or more is required for dividing cementite under the state of poor cold-working properties, and cannot be considered to be a stable manufacturing method.
Furthermore, a method has been proposed for accelerating graphitization by adding B, Al, or a rare-earth metal (REM) such as La and Ce to 0.53% C steel. For instance, see: Iwamoto et al., "Effects of B, Al, and REM on the Graphitization Behavior of 0.53% C Steel", CAMP-ISIJ, 1995, Vol. 8, p. 1378.
Since it has been reported that the average particle diameter of graphite can be decreased to 2.7 .mu.m according to this method, some effects can be considered. However, in this method, the addition of B and Al together is required for realizing the largest number of graphite particles, and the addition of either one alone cannot realize a large number of graphite particles as shown in FIG. 1 of the previously mentioned Iwamoto reference.
As to steel, since it is not easy to control a small quantity of an added element, and the effect of multiple addition is large, there is a problem that this method is not practical.
A common problem of the above graphitization methods is that graphitization must be performed at a high temperature and takes a long time for processing. Complete graphitization requires a temperature of 700.degree. C. and a time between 10 and 50 hours, significantly lowering the manufacturing efficiency.